1. Field of the Invention
The invention generally relates to an apparatus and method for reducing noise and, more particularly, for reducing noise and distortion in a voltage controlled amplifier.
2. Description of Prior Art
A voltage controlled amplifier ("VCA") amplifies an input signal according to the level of a control signal. The VCA is often used in audio consoles in order to control the gain of an input audio signal. By having the audio signals pass through separate VCAs, the levels of the various audio signals, and therefore the mix of the signals, may be selectively controlled by varying the control voltages applied to the respective VCAs. Also, the control voltage may be varied in order to fade the audio signal either in or out.
For example, with reference to FIG. 1, a VCA IC1 receives an input audio signal at its IN+ and IN- input pins. A control voltage is supplied to the VCA IC1 at its VC pin. The VCA IC1 then produces an amplified input audio signal, which is generated at its output pin VG, according to the magnitude of the control voltage. When the control voltage is at 0 volts, the input audio signal would have full attenuation. At a control voltage of 4.44 volts, the input audio signal would have unity gain and at a control voltage of 5 volts, the VCA IC1 operates at +12 dB gain.
The output of the VCA IC1 is applied to a non-inverting input of an op-amp IC2. The op-amp IC2 has a parallel combination of a resistor R4 and a capacitor C14 connected between an output of the op-amp IC2 and an inverting input of the op-amp IC2. The output signal generated at the output of op-amp IC2 is supplied to an output terminal T2. The VCA IC1, however, is prone to DC feed through, has a relatively very high distortion level, and has a very high noise level. For instance, with the prior art configuration of FIG. 1 where the gain of VCA IC1 is set to 0 dB and the gain of op-amp IC2 is set to 0 dB, the VCA IC1 has a noise level of 92.5 dB. The total harmonic distortion ("THD") as a percentage of unity gain for 0 dB, 20 dB, and 21 dB are 0.0041%, 0.0154%, and 0.221%, respectively. The intermodulation distortion ("IM") at 0 dB, 12 dB, 20 dB, and 21 dB are 0.0125%, 0.0242%, 0.0481%, and 0.280%, respectively.